a=\frac{(7^5)^3:(\sqrt7)^{12}}{(49^{3\frac{1}{4}})^2*(\frac{1}{7})^{19}}=
=\frac{7^{15}:7^{\frac{12}{2}}}{49^{\frac{13}{4}*2}*7^{-19}}=
=\frac{7^{15-6}}{(7^2)^{\frac{13}{2}}*7^{-19}}=
=\frac{7^9}{7^{13}*7^{-19}}=
=\frac{7^9}{7^{-6}}=7^{9-(-6)}=7^{9+6}=7^{15}